2000 Chevy Camaro Can't start the engine

I accidentally turned the engine off while stopped on ramps with the front wheels (in order to change the oil), but in drive. I shifted in parking (with engine off) and after I added engine flush (to clean the engine before draining the oil) I tried to start the engine to run it for 15 min as per engine flush instructions. It started but in the next second it stopped. I tried a couple more times, engine turns but it will not fire up. Tried to start it with pedal fully depressed to clean an eventual flooded engine. Still no start, just turning. I then checked the fuses: all good. Swapped the fuel pump relay with a similar one on the fuse block: same symptoms. I jumped the contacts on the fuel pump relay socket to force the pump to run and checked for pressure on the fuel pressure check valve. Not much pressure. Turned the key in start position and checked the pressure again: sometime I will have some pressure but most of the time no pressure at all. I disconnected the battery to reset the computer and tried everything all over again with the same results. With a meter i checked the voltage across fuel pump relay contacts: 8V. Battery was good (drained it now after so many attempts to start), more than 3/4 tank full with gas. Is there a fuel pump cutoff switch or fuel pump voltage check point so that I can check to see if the pump gets the 12V ?
Thanks!!!